The `Mail::Gpg.signed?` method raises an error if it didn't
detect any signature, but detects the PGP boundaries in the
body.

This becomes a problem if folks include any kind of PGP
boundaries for an encrypted message in their body. This can
e.g. happen if you try to describe how PGP works, include
another PGP signed message in your mail (e.g. don't want to
have even schleuder access to your mail content), you
have a MUA with problematic quoting behavior or you simply
forward an email with PGP boundaries, which you e.g. might
have received through Schleuder's admin notification.

This patch addresses these cases by a) checking if the
decrypted mail had any signatures on it and so we already
verified signatures and b) using the detection methods
directly, so we don't run into raising the encryption of
better using `.decrypt`.

2 tests are included to demonstrate the problematic behavior
and which should cover the 2 main cases described in the bug
report.

These changes have the side effect, that we won't anymore verify
any signatures within an email that already had a signature as
part of the encrypted blob. This can be seen, e.g. that we need
to change the test for the `x-sign-this` keyword, as a side-effect
of using schleuder to decrypt the mail sent by schleuder, we had
also the signature of the signed blob stripped away. Actually,
we were testing for the wrong thing in this test anyway.